In many blasting applications it is desirable to have an explosive of reduced and variable bulk strength. In driving tunnels or galleries careful blasting of the contour holes will give a substantially undamaged rock face with strongly reduced needs for subsequent repair and support work such as bolting, gunniting, concrete reinforcement etc. and the final profile will be true the design size. Similar considerations araise in underground mining and stoping as well as in bench blasting to limit production of fines to meet certain after-processing constraints.
Although numerous closely spaced bore-holes can be used to produce smooth fracture planes, the method is limited by practical and economical reasons and conventionally careful blasting has been carried out by partial charging of oversized boreholes with small-diameter cartridges or tubes. Another approach is the arrangement of spatially separated and individually ignited deck charges at regular intervals in the borehole. The methods are expensive and give little variability in energy output. Frequent problems are inconsistency in charging and uncontrolled coupling between explosive and rock. Detonation failures have also been experienced for certain explosives, supposedly due to precompression from forerunning shock waves in the free gas channel. Introduction of shells or spacers concentric with the charge have improved positioning but added to cost and complicated charging procedure.
To meet the general trend towards wider boreholes and bulk charging of explosives also in connection with careful blasting, bulk explosives of strongly reduced energy concentration have been developed, such as ANFO mixed with porous lightweight material.
The complete fill out of large drill holes with explosive places severe demands for energy reduction and the explosive often approaches its detonation limit. Although the positioning problems mentioned in connection with the packaged products are avoid with bulk explosives, the coupling to the rock surface is stronger and the blast result will be markedly dependent on any inhomogenity present in the explosive. The lightweight materials usually employed for energy reduction are susceptible to static electricity and are not easily mixed with the heavier standard components of the explosive. Precautions taken at manufacture to secure thorough mixing are not sufficient since the components tend to separate during transport and charging operation. The cohesion of ANFO and the possibility to charge in vertical upholes are normally dependent on partial destruction of porous ammonium nitrate prills and the ability of the minor amount of fuel to bridge the so created debris. This ability is strongly reduced by the inclusion of substantial amounts of dry inert fillers and known reduced explosives are of limited use in these applications. Attempts have been made to reduce the abovesaid problems by adding water or adhesive agents to the explosive compositions. Water desensitizes the explosive and can only be used in small amounts. Water is not compatible with either the fuel phase or organic extenders and hence of poor cohesive quality and wetted compositions are not stable over prolonged periods. Adhesive or tackifying additives, as examplified in the British Patent Specification 1 311 077, generally acts as fuel in the explosive and deteriorates the oxygen balance unless the regular fuel amounts is reduced proportionally, in which case a less efficient fuel/oxidizer distribution has to be accepted. Under all circumstances operable amounts are very limited, also for obtaining secondary advantages such as water resistance. Organic additives of this kind may also dissolve, deflate or otherwise adversely effect extenders of organic origin.